1. Field of the Invention
The present invention relates to a turbine which can maintain an appropriate clearance between an rotor blade and a shroud during an operation thereof.
2. Description of Related Art
In FIG. 4, an example of a turbine which is used for a gas turbine or a jet engine is known. A turbine 100 comprises a rotor 110 having a plurality of rotor blade unit 111 which are disposed along a rotating shaft X—X intermittently and a cone-shaped shroud 120 as a jacket for the rotor blade unit 111. Reference numeral 121 indicates a stator blade which extends from an inner wall of the shroud 120 so as to be disposed between the rotor blade unit 111. A high-pressure-fluid is introduced in to a turbine 100 from a smaller diameter region of the shroud 120, the rotor 110 rotates due to a force which is generated at screw surfaces of the rotor blade unit 111; thus, it is possible to convert kinetic energy of the fluid into a rotational force. In the turbine 100, a clearance is formed between tips of the rotor blade unit 111 and an inner wall of the shroud 120 facing the tips of the rotor blade unit 111 so as to prevent contact by both of them. If this clearance is too large, the fluid leaks from a higher-pressure region to a lower-pressure region of the rotor blade unit 111; thus pressure-loss occurs and operational efficiency decreases. Therefore, it is necessary to minimize the clearance so as to restrict the leak of the high-pressure-fluid and increase the efficiency of the turbine operation. This applies to a case of a turbine which converts a rotational force into fluid pressure.
On the other hand, if the clearance is too small, the tips of the rotor blade unit and the inner wall of the shroud 120 contact in an initial phase of the turbine operation due to factors such as thermal expansion of the rotor blade unit 111, centrifugal force on the rotor 110, and vibrations of the overall turbine 100. Because of this, the tips of the of the rotor blade unit and the inner wall of the shroud 120 slide against each other when the rotor blade unit rotate. Such a phenomena is commonly called an “initial slide”. Also, if a turbine is operated for longer periods, the rotor blade unit 111 and the shroud 120 are exposed to a high-temperature-high-pressure fluid and thermal expansion occurs. In such a case, it sometimes happens that the tips of the rotor blade unit 111 and the inner wall of the shroud 120 contact and slide. Such a phenomena is commonly called a “secondary slide”.
Commonly, for example, for purposes of heat protection and oxidization, a protecting layer is formed on a shroud and a rotor blade. For the purpose of heat protection, a thermal barrier coating (hereinafter called TBC) made from a zirconium oxide ceramic member is used. Also, for the purpose of oxidization protection, a layer of MCrAlY (M is at least one of Fe, Ni, CoNi, NiCo, and Co) is used. Also, MCrAlY can be CrAlY (in this case, M is nothing). However, hardness of the TBC as the outermost layer is high. Therefore, when the tips of the rotor blade unit 111 and the inner wall of the shroud 120 contact and slide, there is serious damage, particularly to the rotor blade, due to friction heat and sliding stress. In order to solve such a problem, in Japanese Unexamined Patent Application, First Publication No. Hei 4-218698, Japanese Re-Publication of PCT International Publication for Patent Applications No. Hei 9-504340, and U.S. Pat. No. 5,702,574, a gas turbine in which an abrasive layer 112 is formed in a matrix having MCrAlY at a tip of the rotor blade unit 111 is disclosed. FIG. 5A shows an example of such a turbine. In this example, an abrasive particle such as CBN (Cubic Boron Nitride) particle 113 is dispersed in the matrix. In this example of the turbine, the CBN particles 113 protrude.
When the abrasive layer 112 is provided on the tip of the rotor blade unit 111, the tip of the CBN particles 113 which protrude from the abrasive layer 112 grinds the inner wall 123 of the shroud so as to form a groove 124 even if the tip of the rotor blade unit 111 and the inner wall 123 of the shroud 120 slide against each other when the rotor blade unit 111 rotates. This is because hardness of the CBN particle 113 is higher than the hardness of the protecting layer 122 (for example, zirconium oxide ceramic member) of the shroud 120 as shown in FIG. 5B. By doing this, it is possible to obtain an appropriate clearance. Also, in Japanese Unexamined Patent Application, First Publication No. 2000-345809, a gas turbine engine having an abrasive coating which is made by embedding particles such as CBN on the inner wall of the shroud and by protruding it and an abradable coating which is supposed to be ground by the above-mentioned particle and formed on the tip of the rotor blade. In such a case, particles which are disposed on the inner wall of the shroud grinds the abradable coating which is disposed on the tip of the rotor blade unit when the rotor blade unit rotates; thus, it is possible to obtain the clearance. As explained above, in conventional suggestions for attempts to obtain the clearance by a grinding-operation by using rotation of the rotor blade unit, clearance was formed by forming an abrasive surface which is formed by embedding a hard particle such as CBN to either one of the tip of the rotor blade unit or an inner wall of the shroud and an abradable surface to the other, and grinding the abradable surface by the particles on the abrasive surface. However, by such a technique, it was necessary to restore or remake the abradable surface when the turbine is examined for maintenance purposes because the abradable surface is deeply ground because of the initial slide and the secondary slide during the operation of the turbine. For restoring such abradable surfaces, huge appliances such as a blasting apparatus, a thermal spraying apparatus, and a high temperature heating furnace were necessary. Therefore, it was difficult to restore or remake the abradable surfaces at a manufacturing site where the turbine is located. From this point of view, a turbine which can realize the restoration and remaking of the abradable surface easily has been required. Also, when a gas turbine in which a CBN particle is used on the abrasive surface is actually operated, the grinding performance decreases in a temperature over 900° C. due to factors such as deterioration of the CBN particle because of oxidization. It was also found that, when the turbine is operated further, overall CBN particles disappear and the abrasive surface and the abradable surface slide on each other directly. In this case, the abradable surface made from the ceramic member is harder than a matrix layer of the abrasive surface made from MCrAlY. Therefore, it was also found that the abradable surface also grinds the abrasive surface. Furthermore, it was pointed out that the rotor blade unit is possibly exposed and sticks to the abradable surface. The grinding performance of the CBN particle decreases rapidly under high-temperature conditions because it is estimated that the CBN repeatedly oxidizes and sublimites under high-temperature conditions. Therefore, durability is a problem if a gas turbine and a jet engine are operated for longer periods under high-temperature conditions. Therefore, it is thought that a conventional turbine using a CBN particle for forming a clearance is only useful during a period of an initial slide.
Also, as shown in FIG. 5B, if a groove 124 is formed on an abradable surface 123, the clearance becomes too large in an initial phase after the turbine is restarted. In such a case, there is a possibility that the pressure loss which is caused by fluid leak cannot be ignored. Also, if the protecting layer 122 as an abradable surface is made from zirconium oxide ceramic member, huge apparatuses and different restoring techniques were necessary to restore or remake the groove 124 because it was necessary to form an intermediate layer on a base material by a thermal spraying method and further form a zirconium oxide ceramic layer in a uniform thickness. Such operations could not be performed in a site where the turbine was located.